1. Field of the Invention
This invention relates to a method of testing an optical receiver comprising DC (Direct Current) coupled circuits for an optical link or an optical interconnection used in an information processor, and more particularly to a method and apparatus for testing the characteristic of a DC coupled optical receiver when a bit error rate of the receiver is very low.
2. Description of the Related Art
A optical receiver will be described hereinafter.
In a conventional receiver, a small optical signal, received after a long distance transmission through optical fibers, is converted to an electric signal, which is regenerated after being amplified. To stabilize the circuit operation, an analog amplifier of the receiver cuts a DC component of the signal.
The optical signal is coded and scrambled such that an average DC level is set to be constant within the amplitude of the signal (usually a mid-level of the amplitude of the signal). As used herein "DC level of light" or "DC light level" means a constant intensity level of light. The decision level (threshold) of 0 or 1 in the receiver is set with respect to the average DC level. To this end, the receiver has a complicated structure which includes an automatic gain control circuit which provides a fixed amplitude using amplitude detection and gain control.
However, optical links provided between information processors, such as computers or switching systems, are required to transfer uncoded data, so-called unformatted data. The devices used for such optical links are required to be reduced in size and have reduced power consumption and the circuits in the devices are required to be simplified. The above-mentioned requisites are indispensable for optical interconnection, such as is disclosed in Atushi Takai et al.: "Sub-system optical Interconnections using Long Wave-length Laser Diode and Single-mode Fiber Arrays", Institute of Electronic, Information and Communication Engineers of Japan, TECHNICAL RESEARCH REPORT OCS92-30, published in 1992.
One type of optical receiver which satisfy such requisites includes DC coupled amplifiers, which do not cut DC component of the signals. In the above-mentioned case, the transmission distance is several hundred meters, so that transmission loss is negligible and an optical signal having a large amplitude can be input to the receiver. Thus, a slight fluctuation in the threshold, which distinguishes between 0 and 1, is allowable. In addition, the number of amplifier stages is reduced. Furthermore, since in the above-mentioned case a deviation in the input optical power is large, due to variations in the transmitter optical output and in the connecter loss, which causes the average DC level fluctuate, the DC coupled optical receivers are employed using an extinction level as a reference for defining a threshold in many cases.
As described in the reference, such an unformatted data receiver adopts a fixed decision level relative to the extinction level which does not depend on the amplitude of the input signal. This simplifies the circuit structure, which thereby reduces the size and power consumption of the optical receiver.
Of course, the DC coupled optical receiver is applicable to the transfer of coded or scrambled data, although the applicable distance is short compared to the conventional receiver.
A method of testing the optical receiver will be described next. Conventionally, when the bit error rate characteristic is tested, the input optical signal to the optical receiver is attenuated by an optical attenuator and the relationship between the amplitude of the signal and bit error rate is measured, because the circuit and the threshold are designed to use a given level within the amplitude (usually, a midpoint of the amplitude) as a reference and waveform degradation in the receiver, which will influence the bit error rate, is substantially directly proportional to the amplitude of the signal.
In contrast, since the DC coupled receiver uses the extinction level as a reference for defining the threshold, a new system which measures the bit error rate characteristic is required to be devised. Waveform degradation in the receiver at the extinction level is required to be directly measured because the bit error rate depends on the waveform at the extinction level.
In a multi-channel receiver, an influence by a cross-talk depends on the phase difference in data between a reference channel and other channels in the optical interconnection. Thus, cross-talk at the extinction level is required to be directly measured.
Further, a low bit error rate, on the order of 10.sup.-20, must be ensured to apply the DC coupled receiver to an information processor and thus a testing method therefor must be established.